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Like pretty much anything, they're scattered around the Wasteland. You can always buy them (trade useless junk like cans to offset the cheap price if you really dont want to spend caps). The other confirmed spots are on the bodies of Mercs/Raiders/Wastelanders you kill. Not alot of them, but a fair percentage.

All of the above answers are true, but there is no specific answer as to "where to find bobby pins", your safer bet is just to search every metal box, ammo box, first aid kit, and pretty much any container you find, there are lots scattered around the wasteland...

And a tip to save some of those, don't rush into picking locks, do it calmly and slowly, and you won't break much of those ;)